That's 9%/year annual rate of return (assuming 2% inflation), which is good, but not unheard of (here's the calculator i used. This ignores all the money they put into the place while they lived in it (roof's and heaters etc aren't cheap). It also discounts that they need to find a place to live in 2021 where all houses just got much more expensive then back in 1986.

That whole time they were paying property tax.